It goes back to proto west germanic thuris, and that goes back to proto germanic Thurisaz, which means a giant or a monster. I think it would be cool to see this word in the dictionary. This word Thurisaz itself comes from PIE **tur- or twer-
In doing an extensive literature review for my draft work on runelore, I have never encountered thurse in any work in English, Middle English or Old English, but of course, my readings have not been exhaustive. That said, I have encountered thurs (singular) and thurses (plural).